A simple method for the preparation of 2,4,6-trichloroborazine and evaluation of 1H-NMR spectra

Abstract

ABSTRACT: The compound 2,4,6-trichloroborazine (TCB) is known as a valuable precursor to prepare boron nitride and various derivatives of borazine. Difficulties in the preparation of TCB via conventional methods on the one hand, and its importance for the material science, on the other hand, were our motivation to modify the Brown’s method for TCB synthesis. So far, all reports on the synthesis procedure of TCB are based on the use of gaseous BCl3 with NH4Cl powder at 110-130 °C in chlorobenzene requiring complicated low-temperature equipment such as a cold finger condenser, and dry ice-acetone reflux condenser to retain BCl3 gas in the reaction flask. According to our study, using BCl3 solution in n-hexane was more convenient than using gaseous BCl3. The reaction of BCl3 solution with NH4Cl was carried out without loss of BCl3. Furthermore, the reduction in the reflux temperature from 130 °C in some previous studies to 86 °C in our study, was a valuable advantage, leading to the release of the coordinated BCl3 from the adduct complex [CH3CN·BCl3] gradually and the prevention of thermal decomposition of TCB, unwanted polymerization reactions and the formation of undesirable products, resulting in the increased reaction yield. TCB was fully characterized by ATR-FTIR, 11B-, 14N- and 1H-NMR spectroscopic methods. 1H-NMR spectra of TCB were performed for the first time at various temperatures to elucidate the quadrupolar effect of nitrogen, which led to evaluate the splitting of the proton resonance of the 14N-H bond by coupling with the 14N quadrupole nucleus. The broad peak observed at room temperature was clearly split into a triplet at 100 °C due to the long TqN and fast molecular motion of TCB molecules. The 14N-NMR spectrum was also shown for the first time as a broad signal at δ = –271.3 ppm (h1/2 = 186 Hz).